how can the universe be expanding if matter nor mass can be created?
Because the expansion doesn't involve creating matter. It's just existing matter getting further apart.
That said, matter/mass can certainly be created or destroyed. Mass is a form of energy, and other forms of energy can be converted to mass under the right conditions. When heavy particles decay or matter and anti-matter meet and are annihilated, mass is also destroyed.
The truth, there is no satisfying explanation. They really don't understand the mechanics behind how space expands...
Separate names with a comma.